The invention relates to a circuit arrangement for an electric drive, a method for its operation and a motor vehicle comprising the circuit arrangement.
For supplying electrical energy in vehicles there are various energy storage systems. Useful are, for example, double layer capacitors with a high power density and low energy density, batteries with average power and energy density, as well as fuel cell systems with low power density and high energy density.
These energy storage systems are used predominantly in a single source mode in hybrid and/or electric vehicles. The prior art discloses proposals that treat the respective limitations of the accumulators with respect to the power and energy density by means of a parallel or series circuit of different kinds of accumulators. In this case a coupling of the different kinds of accumulators can be controlled by means of switches and/or power electronic valves as a function of the state of charge of the individual accumulators.
There are also circuits that connect together the power circuits of various voltage levels and influence the energy exchange between these power circuits. Typically, DC/DC converters are used to this end.
The known strategies have the drawback that when the motor vehicle, exhibiting a noteworthy cruising radius and in a cycle with practical acceleration values, is running exclusively in electric mode, an energy efficient operation is not possible. Vehicles with double layer capacitors achieve outstanding acceleration values with simultaneously low accumulator weight and high efficiency, but have permitted to date only a few hundred meters travel in electric mode. Vehicles with batteries make possible travel of several kilometers in electric mode. However, the losses occurring in the batteries at higher power conversions (air conditioning) can be compensated only with a lot of engineering effort and lead to a very low system efficiency. Vehicles with a fuel cell drive are currently not cost effective owing to the high power output costs and the high power-to-weight ratio.
A parallel circuit of a double layer capacitor and a battery results in an additional energy conversion between the two accumulators (power circuits for charge balancing in the case of voltage asymmetry) and, thus, to a reduction in the system efficiency.
A series circuit of capacitors and batteries leads to additional complexity in the control of the state of charge. Both types of accumulators differ in their self-discharging behavior. In order to compensate, an active recharging or a discharging of at least one portion of the accumulators is necessary, as a result of which the system efficiency is also significantly decreased.
The object of the invention is to avoid the aforementioned drawbacks and to provide, in particular, a strategy for decoupling various energy accumulators while simultaneously taking into consideration any energy balancing processes and, in particular, to ensure high efficiency in the course of supplying an electric drive by means of several different energy sources.
This and other objects are achieved with a circuit arrangement and method for supplying an electric drive, to which at least two electric energy sources can be connected. At least one of the at least two electric energy sources supplies at least intermittently the electric drive by way of at least one actuating element. At least one electric energy source can be disconnected from the electric drive by way of a switch.
With this strategy it is possible to combine together different or multiple identical electric energy sources including, for example, electric buffers and/or electric energy accumulators for the purpose of supplying an electric drive. The energy sources can differ significantly with respect to power and/or no-load voltage. In fact, the electric drive can be supplied even as a function of the respective properties or operating points of the electric energy sources that are used. The switch is constructed preferably as an electric switch.
At this point it must be pointed out that the electric energy source can also include various electric energy accumulators. Furthermore, it must be pointed out that the batteries mentioned herein are, in particular, rechargeable.
In particular, the electric energy source may also be an energy supply system, a generator, solar cells or thermoelements. Similarly it is possible that, instead of the electric drive, another electric consumer or another electric source and/or an electric supply system with at least a slightly inductive behavior is used. At the same time the inductive behavior can be inherently present or given by means of additional elements, for example, a throttle. In particular, the number of phases of the electric drive (of the consumer or the source, etc.) as well as their fundamental electric frequency is irrelevant for the operating principle.
It is a further development that the switch is designed so as to be uncontrolled and comprises, in particular, a diode.
It is an additional further development that the switch is a controlled switch comprising at least one of the following components:
a transistor;
an IGBT [isolated gate bipolar transistor];
an FET [field effect transistor]; in particular, a MOSFET [metal oxide semiconductor field effect transistor];
a thyristor;
a triac; and/or
a dynistor.
It is, in particular, a further development that at least those electric energy sources that exhibit a direct voltage that is lower than that of the other energy sources can be disconnected from the electric drive by means of the switch. The direct voltage can be the no-load voltage of the electric energy source.
Correspondingly, the electric energy source that originates from the at least two electric energy sources and that exhibits a higher direct voltage or no-load voltage than the other electric energy sources can be connected permanently (thus, without an active switch) to the electric drive.
It is also a further development that the electric drive comprises an electric machine. In particular, the electric drive can have a plurality of electric machines. For example, the electric machine can be constructed as a multi-phase electric machine. Preferably one or more electric machines can be provided in a motor vehicle.
Furthermore, in one aspect the at least two electric energy sources are disposed with the circuit arrangement in a motor vehicle.
In particular, the strategy described herein can be added to a motor vehicle with an internal combustion engine.
In accordance with an additional further development, the electric energy source comprises one of the following energy sources:
a fuel cell;
a battery; or
a capacitor source, in particular, a double layer capacitor source.
A next further development consists of the actuating element comprising an inverter bridge. In particular, the inverter bridge (also called the inverter) can be designed so as to support three phases. In one embodiment each electric energy source has an inverter bridge. As an alternative, a single inverter bridge can be provided for the at least two electric energy sources.
An alternative embodiment consists of providing a buffer to absorb and/or make available electric energy. The electric buffer is disposed preferably in a freewheeling path so that excess energy can be absorbed especially from the electric drive and, if desired, consumed. In particular, one of the electric energy sources can be designed as an electric buffer.
In another embodiment the buffer exhibits a voltage that is essentially at least as large as the largest voltage—especially the largest no-load voltage—of the electric energy sources.
In yet another embodiment the buffer exhibits a voltage that is essentially at least as large as the maximum voltage of the electric drive.
A further development consists of configuring the buffer in such a way that it absorbs the energy that is stored inductively in the electric drive and releases this energy to either the electric drive or another consumer.
The electric buffer can be designed as any possible variant of an electric energy source (for example, as a capacitor, battery, especially in combination with a varistor or the like).
One embodiment consists of connecting in series at least two electric energy sources. This approach makes it possible to use in an advantageous way various energy sources exhibiting different power levels.
It is another embodiment that the at least two series-connected electric energy sources can be activated individually or together. In this way a multi-level strategy for different voltages can be made available by use of the at least two electric energy sources. Depending on how the electric energy sources are connected to the system, only one energy source can be, for example, active, or both energy sources can be active. The term “active” is understood herein to mean that the energy source supplies the electric machine or receives energy from the electric machine.
For example, different voltage levels can be made available for the electric machine in that in a first state only one of the two energy accumulators supplies the electric machine with energy, and in a second state both energy accumulators supply the electric machine with energy. For example, the energy accumulator that is activated in the second state can make available a high voltage for a short period of time, in order to complement (boost) the basic supply of the other energy accumulator.
It is also a further development that the at least two series-connected electric energy sources can be activated individually or together by means of at least one electronic switch.
Another further development consists of providing a changeover switch, by which it is possible to switch between the electric energy sources. For example, it is possible to switch between a first group of energy sources and a second group of energy sources in such a way that each group has at least one energy source. In particular, if one group is activated due to the switching operation, then the other group is deactivated. It is also possible that the changeover switch switches between several groups of energy sources (multiport switch).
In addition, it is a further development that at least one electric energy source is a mono-directional energy source. As a result, the mono-directional energy source can be provided either only for supplying the electric machine, or the electric machine can transmit only energy into the mono-directional energy source. It is also possible that an energy source is configured so as to be temporarily mono-directional (for example, by use of a switchable diode).
In another development the at least two electric energy sources exhibit different voltages and/or different power outputs.
Moreover, the invention may be provided in a motor vehicle having at least one electric drive and at least two electric energy sources, as well as the circuit arrangement, described herein.
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of one or more preferred embodiments when considered in conjunction with the accompanying drawings.